1. Field of the Invention
The present invention relates to a method and apparatus for high throughput holographic microlithography in which interferometric patterning techniques suitable for producing periodic arrays of sub-micron sized structures are adapted for and incorporated into a high-throughput, large field size manufacturing tool. The method and tool of the present invention have applications in the display, semiconductor, and optics manufacturing industries.
2. Description of the Prior Art
An unmet need exists for an efficient tool adapted for production of flat panel displays based on distributed cathode field emission display (FED) technology, a strong competitor in the flat panel display market currently dominated by liquid crystal display (LCD) manufacturers. A FED is a distributed cathode, flat panel analog to the well known cathode ray tube (CRT). Essentially, billions of miniature electron `gun` cathodes are distributed spatially over the surface of a display substrate. Electrons are emitted from the tiny cone-shaped cathodes under the influence of a high accelerating potential, and strike a phosphor screen placed over a common anode and are thereby converted to photons (i.e., light). The most critical step in the fabrication of the FED distributed cathode matrix is patterning of an array of holes or wells in which emitter cones are grown. In the prior art, a photosensitive medium such as photoresist has been employed to record an image of a hole array formed by conventional photo- lithographic techniques such as shadow masking (contact printing), optical projection, electron or laser beam direct writing. The hole array or pattern, in photoresist, can then be used as an etch mask in the process of forming the holes. In the prior art, hole patterns have been limited by resolution and field size of the imaging or writing systems, and complex, often expensive, work-around solutions have been required to achieve modest field sizes of fifty by fifty mm with hole diameters in the one to two micron range. Recent research has demonstrated that reduction in the hole size (and consequently the emitter size), below the one micron range provides numerous benefits such as a reduced gate voltage, lower power consumption, greater current densities per pixel, and built-in redundancy. Thus, to fully realize the potential of FED technology, an inexpensive, high speed, production environment lithographic tool, incorporating a patterning technology capable of producing large-area, high-density, sub-micron diameter hole arrays with few defects and at low cost, is needed.
Holographic or interferometric lithography has been proven in laboratory environments to be feasible for generating the high-resolution periodic structures suitable for flat panel FEDs and exploits the mutual coherence of multiple optical beams derived from a single light source such as a laser. The laser beams are made to overlap in some region of space and interfere to produce patterns of light and dark areas that repeat on a scale proportional to the wavelength and are subsequently recorded in photosensitive media such as photoresist. Conventional contact or projection photo masks are not required and so holographic lithography is known as a "maskless" lithography technique. In addition, by exploiting inherent photoresist and etching process non-linearities, a variety of surface relief structures can be generated with no change in the optical configuration.
Other useful surface relief structures can be patterned using holographic lithography such as a "motheye" or sub-wavelength-structure (SWS) surfaces. Motheye surface structures have been shown to be effective for nearly eliminating the reflectance of light from an optical interface such as between air and a window or a refractive optical element. The term "motheye" is derived from the insect's eye, a natural analog; it was observed that the eye of a nocturnal insect (e.g., a moth) reflected little or no light regardless of the light wavelength or the angle at which incident light struck the eye surface. The eye surface functions in a manner similar to a graded index material, essentially allowing the smooth transition between media with differing bulk density. To avoid diffraction effects, synthetic motheye surfaces must be fabricated with feature sizes and spacings smaller than the wavelength of light incident upon the surface. For most infrared or visible wavelength applications, this necessitates structure spacings in the sub-micron to sub-half micron range, patterned over the entire surface (e.g., window or optic area). A means for manufacturing motheye structures in high volumes and over large areas is not available in the prior art and a variety of products could benefit from the increased ruggedness and anti-reflective performance afforded by motheye surfacing over large areas.
Manufacture of liquid crystal displays (LCDs), also requires improvement. Liquid crystals (LCs) are anisotropic molecules which can affect the properties of light with which they interact, and, under the influence of an electric field, can vary the magnitude of this affect. LCDs are formed by the creation of a cell, typically constructed using glass, within which the LC molecules are confined. The term "crystal" refers to the structure or ordering of the LC molecules into a definable or measurable state typically found with molecules in a solid state. This artificially created ordering is accomplished by depositing thin layers of material on the boundaries of the cell, which either physically, or electrostatically force the LC molecules to preferentially align in one direction. The "alignment layers" as they are known in the art, are typically processed using a physical rubbing or buffing technique comprising a spinning drum or cylinder and rolling it over the cell substrate coated with alignment material. High levels of hazardous static charge and spreading particulate (from the rubbing material) are generated during this process; in addition, manufacturing yields can be improved.